1. Field of the Invention
The present invention relates to a groove structure of a wet-type friction engaging element.
2. Discussion of the Prior Art
Wet-type friction engaging elements have been used for example in an automatic transmission for motor vehicles. As the wet-type friction engaging element of the automatic transmission, there have been used a friction plate, a brake band, etc. in a multiple disk friction engagement device. As shown in FIG. 8, a multiple disk friction engagement device 20 conventionally has a hydraulic-actuated piston 21, a plurality of wet-type friction plates 22 arranged alternately so as to be engaged with each other by a piston 21, and a mating plate 23. The wet-type friction disk 22 has a core plate 24 and friction members 25, 25 fixedly attached on both sides of the core plate 24.
As shown in FIG. 7, in the surface 26 of the friction member 25 which is in contact with the mating plate 23, a plurality of radial grooves are usually formed. The grooves are so formed as to improve the coefficient of dynamic friction during initial engagement (coefficient of dynamic friction of initial engagement is herein designated .mu.i) of the multiple disk friction engagement device 20 by breaking a film of cooling oil interposed between the friction member surface 26 and the mating plate 23 when the friction member surface 26 is pressed into contact with the mating plate 23.
The cross-sectional shape in the direction of width of a conventional groove 27, shown in FIG. 9, is a circular arc form of a greater depth at the center. In the present invention, the "direction of width" identifies the direction which intersects at right angles with the longitudinal direction of the grooves and is parallel with the surface of the friction member.
A problem with the grooves 27 which are of a circular arc form depressed at the center is that a corner section 28 at the border between the grooves 27 and the friction member surface 26 becomes shallow, so that when the mating plate 23 and the wet-type friction plate 22 are pressed into contact with each other, the oil pressure at the corner section 28 will become higher than that at the central part as shown by the pressure versus distance curve in FIG. 9. The cooling oil is forced by this high oil pressure in between the mating plate 23 and the friction member 25 through the grooves 27, making it hard to break the oil film and accordingly lowering the coefficient of dynamic friction during initial engagement .mu.i of the multiple disk friction engagement device 20. This problem arises particularly when the contact pressure between the mating plate 23 and the wet-type friction disk 22 has been set low and when the multiple disk friction engagement device 20 is used at low temperatures.
To obviate the above-described problem, the adoption of grooves 30 having a U-shaped cross-sectional shape in the direction of width as shown in FIG. 10 is considered. The groove 30 is formed deep at both corner sections 32, 32 of the bottom in the direction of width, and the oil pressure is lower at the corner sections 31, 31 (the upper corner sections of the groove) at a boundary between the friction member surface 34 and the grooves 30 than at the corner section 28 of the grooves 27 in FIG. 9. Accordingly, this makes it difficult to force the cooling oil in between the mating plate 23 and the friction member 35 of the wet-type friction plate 33. The oil film, therefore, is broken early, thereby enabling improvement of the coefficient of dynamic friction during initial engagement .mu.i of the multiple disk friction engagement device 20.
The groove 30 described above, being formed deep as a whole, presents a problem by increasing the sectional area, which requires a greater compressive force in press working to form, resulting in a higher machining cost of the wet-type friction plate 33. Comparing the cross-sectional area S1 in the direction of width of the groove 27 in FIG. 9 and the cross-sectional area S2 in the direction of width of the groove 30 in FIG. 10, if b1 and b2 are the groove widths and h1 and h2 are the depths of the grooves at center in the direction of width, then S1 =(2.cndot.b1.cndot.h1/3) and S2=b2.cndot.h2. Since b1=b2 and h1=h2, S1&lt;S2. Therefore, the cross-sectional area in the direction of width of the grooves 30 is larger than that of the grooves 27.
The groove 27 can be formed deeper than the groove 30 under the same compressive force of a press as seen from the machining data of a press, which is shown in FIG. 6. That is, a greater compressive force is required to form the groove 30 than the groove 27 when these grooves are of the same depth.